In one form of a method of determining a restoring moment produced by tangential forces in a tire testing operation in a tire testing machine, a pneumatic tire is brought into contact with a test surface during a measuring run, and a rolling movement is produced as between the tire and the test surface. The area at which the tire is in contact with the test surface is referred to as the contact surface or contact patch and is sometimes also designated the footprint of the tire. The tangential force measurement values which are obtained in such a measuring run are used to form the restoring moment. The restoring moment is also referred to in the art as the torque steer effect. A specific procedure of that kind as is to be found in German patent specification No. 2 326 046 provides that, in the testing of pneumatic tires and in particular motor vehicle tires, measurement of the tangential forces produced is employed to determine the restoring moment which acts in the contact surface about an axis passing through the center of the contact surface, perpendicularly to the contact surface. A tire testing machine which operates on the basis of that procedure includes a test surface which is in the form of a rotatable test drum and inter alia force measurement transducers disposed on both sides of the test drum at the axis thereof. The force measurement transducers are arranged at equal spacings from a central plane of the test drum, which extends through the axis of the test drum normal thereto. The two transducers supply measurement signals proportional to the tangential forces which produce an effect in the tire contact surface during a measuring run. A signal which is proportional to the restoring moment is formed by multiplication with a signal corresponding to the spacing of the force measurement transducers from the central plane of the test drum, with a difference signal, for the two force measurement signals supplied by the transducers.
However that procedure does not take account of the fact that the contact surface or patch of the tire may be disposed in a displaced position relative to the central plane of the test drum which extends normal to the axis of the drum. That is a situation which may occur in many circumstances of use. That asymmetrical positioning of the contact surface of the tire in relation to the test drum results in a basic restoring moment which is superimposed on the measurement values. Accordingly there are situations in which the operating procedure results in the apparent measurement of a restoring moment, even when there is in fact no such restoring moment, in relation to the center of the pneumatic tire contact surface. That falsification of the measurement result is due to the fact that the tangential force of the tire does not act exactly in the middle of the spacing between the two force measurement transducers, that is to say symmetrically with respect to the central plane of the test drum.